1.1 Technical Field
The present invention is directed to making cellulosic fiber-containing fabric wrinkle-free/resistant by heat-curing using aqueous formaldehyde. More particularly, it relates to making a cellulosic fiber-containing fabric wrinkle-free/resistant by a heat-curing process ("durable press" process) using aqueous formaldehyde and catalyst under conditions whereby the moisture content of the fiber and the reduction of hydrogen bonds in the fiber can be controlled.
1.2 Background Art
In recent years, vapor phase formaldehyde cross-linking, with retention of water absorbency and natural softness, of cellulosic fiber-containing fabric, has been commercialized and proven to be advantageous compared to amino-plastic resin cross-linking. However, despite many attempts, aqueous formaldehyde cross-linking has not been successfully commercialized because of the prior inability to control precisely the formaldehyde content in the fabric due to its evaporation with water during the heat-curing process, as compared to no evaporation of amino-plastic resin during the heat-curing process.
For example, U.S. Pat. No. 4,108,598 describes an aqueous formaldehyde cross-linking process which employs a binder or thickening agent to prevent substantial loss of formaldehyde during curing, or alternatively, with heat-curing under a gradual temperature increase from low temperature to prevent substantial loss of formaldehyde after imparting a moisture content to the fabric of more than 20% by weight. However, such prior art process does not enable sufficient control of the moisture content in the fabric during the curing process. All commercial heat-curing or heat-setting equipment is designed for man-made fiber heat-setting and/or amino-plastic resin heat-curing, neither of which requires the control of moisture and temperature in several chambers precisely as does aqueous formaldehyde cross-linking.
The loss of moisture content in the fabric which occurs in prior art curing processes represents a serious drawback, namely, substantial strength loss due to the increase of hydrogen bonds or hydrogen bonding in the cellulosic fibers which accompanies the gradual decrease of moisture content in cellulosic fiber during heat-curing.
None of the prior art processes for aqueous formaldehyde cross-linking with catalysts, as described in the aforementioned U.S. Pat. No. 4,108,598, as well as in U.S. Pat. Nos. 2,243,765; 3,663,974; and 3,841,832; British Patent 980,980; and Masuda et al., "Textile Finishing Technology" pages 6-142 (1989), has focused on the phenomenon of hydrogen bonding in cellulosic molecules or hydrogen bonding between water molecules and the cellulose molecules in the fiber, the control or elimination of which substantially and advantageously reduces the strength loss of the fabric. Such hydrogen bonding generally has three manifestations: inter-hydroxyl hydrogen bonding between hydroxyl groups on different cellulosic molecules, intra-hydroxyl hydrogen bonding between hydroxyl groups within the same cellulosic molecules, and hydrogen bonding between water molecules and hydroxyl substituents on cellulose molecules.
Although a hydrogen bond is not as strong as a covalent bond, it does have a bond strength of about 5 Kcal/mol. The collective presence of many hydrogen bonds in cellulosic fiber accounts for a substantial loss of tear and tensile strength and abrasion resistance. The reduction of hydrogen bonding in cellulosic fiber can be achieved either by aqueous wetting or by liquid ammonia treatment in a pre-treatment step, and maintaining enough moisture content to prevent an increase in hydrogen bonding caused by drying at curing.
Masuda et al., in Textile Finishing Technology, p. 116, report typical examples of the reduction of hydrogen bonding by mild cure (steam cure) as follows:
TABLE 1 __________________________________________________________________________ Increase of tear, tensile and abrasion strength by preventing the increase of hydrogen bonding in cotton fabric using steam cure Wrinkle-Resistance Weight loss Resin Warp + Filling, angle by abrasion Strength retention (Warp, %) Moisture Type Curing Dry Wet (weight %) Abrasion Tensile Tear Content (%) __________________________________________________________________________ A Steam Cure 259.degree. 274.degree. 4.0 60 97 67 7.5 63.8.degree. C., 5 min B Dry Cure 279.degree. 280.degree. 6.3 10 57 52 3.8 160 C., 3 min Control -- 196.degree. 191.degree. 0.7 (960 times) (23 kg) 1153 g __________________________________________________________________________ 6.8
A: 20% DMMC; 1.2% catalyst (MgCl.sub.2.6H.sub.2 O 50%, citric acid 50%); 2% polyethylene; 0.1% wetting agent.
B: 10% DMMC; 3% catalyst (MgCl.sub.2.6H.sub.2 O); 2% polyethylene; 0.1% wetting agent (Masuda et al., Textile Finishing Technology p. 116).
Table 1 shows to a great extent the remarkable effectiveness of steam curing with moisture in reducing strength loss while keeping high levels of durable press. However, this steam cure method has not been applied to catalytic, aqueous formaldehyde cross-linking of cellulosic fiber-containing fabrics.
In recent years, European mills have been using moist cure to prevent the increase of hydrogen bonds which would otherwise occur by drying at curing. A typical formulation, reported by Cotton Inc. is as follows:
TABLE 2 ______________________________________ Moist Cure Crosslinking Formulation % on weight of bath ______________________________________ Wetting Agent 0.1 DMDHEU (40%) 20 HCl (Conc.) 4.0 ______________________________________ Alkalinity 0.05% or less Moisture 6-8% Batch cure 15 to 24 hrs
Masuda et al., supra, report moist cure which imparts wrinkle-resistance in both dry and wet states, while preventing or inhibiting the increase in hydrogen bonding caused by drying at curing. However, the prior art does not teach the use of aqueous formaldehyde cross-linking with catalysts for moist cure instead of amino-plastic resin.
In summary, the prior art suffers from two major drawbacks:
(1) No prior art method using aqueous formaldehyde cross-linking with catalysts has been able to achieve precise control of the moisture content in cellulosic fiber to secure an adequate level of formaldehyde by preventing its evaporation with moisture at curing.
(2) No prior art method using aqueous formaldehyde cross-linking with catalyst has recognized the hydrogen bond-reduction curing process under which there is precise moisture control to reduce substantial strength loss by preventing the increase of hydrogen bonding in the extent of hydrogen bonding brought about at curing.
The present invention solves the above two problems simultaneously, which thereby also obviates the difficulty of applying catalytic formaldehyde cross-linking to 100% cotton light weight fabrics so as to retain enough tear strength, tensile strength and abrasion resistance to render such fabrics optimally marketable.